The present invention relates to drilling pipe used in directional underground drilling, and the like, and in particular to an improved drilling pipe and method of fabrication of drilling pipe.
Directional drilling permits a bore hole to be cut through the earth with a curved trajectory. In such drilling, a steerable bit is attached to one end of length of drilling pipe. The drilling pipe is driven by a surface motor to rotate the bit and to provide a downward cutting pressure on the bit. The bore of the drilling pipe conducts water or drilling mud to the bit to clear it of earth and rock.
As the bit progresses beneath the surface of the earth, additional drilling pipe is attached to the exposed end of the previous drilling pipe to create a drilling string of increasing length. In order to allow connections of the drilling pipe to each other, the ends of the drilling pipe have threaded couplings. The threaded portions of the pipes are inset from the outer diameter of the pipe, necessitating an increased thickness of the pipe in the end regions.
One method of providing for this increased thickness is a forging of the pipe ends to greater wall thickness followed by a machining of the threads directly to the pipe. This process is expensive and time consuming.
In a second method, a separately machined and coupling is welded to the pipe to provide for the necessary thread interfaces. Normally a friction welding technique is used for this purpose to provide a high weld strength.
In friction welding, the coupling and pipe are rotated about their axes with respect to each other and then pressed together at an interface where the weld is to be formed. The friction between the parts generates heat and removes surface oxide providing for a fusion between the two materials. Weld flash produces a slight bulging at the interface between the two parts which may then be removed by the grinding operation.
In directional boring, the drill string may follow a curved radius on the order of 40 feet. When operating with such curved trajectories, the rotation of the drill string produces constantly changing stresses on the drilling pipes frequently causing failure of their welds.